Method and apparatus for mixing fluids for particle agglomeration

ABSTRACT

An aerodynamic agglomerator ( 10 ) promotes mixing and agglomeration of pollutant particles in a gas stream, to facilitate the subsequent removal of the particles from the gas stream. The agglomerator ( 10 ) is mounted in a duct ( 11 ) through which the gas stream flows. The agglomerator ( 10 ) comprises a plurality of parallel plates ( 12 ) which extend in the overall direction of flow of the gas stream, and are spaced transversely across the width of the duct ( 11 ) to divide the duct into multiple parallel passages. The duct ( 11 ) is configured and/or has formations therein for creating large scale turbulence in the gas stream upstream of the passages. A vane assembly ( 13 ) is provided in each passage for generating a zone of small scale turbulence of such size and/or intensity that the pollutant particles are entrained in the turbulence. Each vane assembly ( 13 ) is located centrally relative to its respective passage and comprises a plurality of sharp-edged vanes ( 15 ) spaced successively in the overall direction of flow of the gas stream. The large scale turbulence in the substreams causes each substream to pass through the zone of small scale turbulence in its respective passage so that particles therein are subjected to the small scale turbulence.

This invention relates generally to method and apparatus for mixingfluids for particle agglomeration. The invention is particularly, butnot solely, suitable for use in pollution control to remove pollutantfine particles from air streams.

In a preferred embodiment, the invention is directed to aerodynamicparticle agglomeration in which particle scale turbulence is used topromote interactions and agglomeration of the particles, and therebyfacilitate subsequent filtration or other removal of the particles fromthe air streams.

This application claims priority from Australian patent applicationsnos. 2003902014 and 2004900593, the disclosures of which areincorporated herein by reference.

BACKGROUND ART

Many industrial processes result in the emission of small hazardousparticles into the atmosphere. These particles often include very finesub-micron particles of toxic compounds. As these fine particles areable to enter the human respiratory system, they pose a significantdanger to public health. The identified combination of toxicity and easeof respiration has prompted governments around the world to enactlegislation for more stringent control of emission of particles lessthan ten microns in diameter (PM10), and particularly particles lessthan 2.5 microns (PM2.5).

Smaller particles in atmospheric emissions are also predominantlyresponsible for the adverse visual effects of air pollution. Forexample, in coal burning installations, stack opacity is largelydetermined by the fine particulate fraction of the fly ash because thelight extinction coefficient peaks near the wavelength of light which isbetween 0.1 and 1 microns.

The importance of fine particulate control can be appreciated byconsideration of the number of pollutant particles in an emission ratherthan the pollutant mass. In fly ash from a typical coal combustionprocess, pollutant particles less than 2 microns in size may amount toonly 7% of the total pollutant mass, yet account for 97% of the totalnumber of particles. A process which removes all the particles greaterthan 2 microns may seem efficient on the basis that it removes 93% ofthe pollutant mass, yet 97% of the particles remain, including the morerespirable toxic particles.

Various methods have been used to remove dust and other pollutantparticles from air streams. Although these methods are generallysuitable for removing larger particles from air streams, they areusually much less effective in filtering out smaller particles,particularly PM2.5 particles.

Many pollution control strategies rely on contact between individualelements of specific species to promote a reaction or interactionbeneficial to the subsequent removal of the pollutant concerned. Forexample, sorbents such as activated carbon can be injected into thepolluted air stream to remove mercury (adsorption), or calcium can beinjected to remove sulfur dioxide (chemisorption). Additionally,particles can be made to agglomerate into larger particles bycollision/adhesion, thereby improving the collectability of theparticles, or the physical characteristics of the individual particlesare otherwise changed to those of an agglomerate which is easier tocollect and/of filter.

However, in order for these interactions to take place, the species ofinterest must be brought into contact. For many industrial pollutants instandard flue ducts, this is difficult for several reasons. For example,the time frames for reaction/interaction are short (of the order of0.5-1 second), the species of interest are spread sparsely (relative tothe bulk fluid) through the exhaust gases, and the scale of the flueducting is large compared to the scale of the pollutant particles.

Normally, exhaust gases from the outlet of an industrial process are fedinto a large duct which transports them to some downstream collectiondevice (e.g. an electrostatic precipitator, bag filter, or cyclonecollector) as uniformly and with as little turbulence/energy loss aspossible. Such turbulence as is generated en route is normally a largescale diversion of gases around turning vanes, around internal ductsupports/stiffeners, through diffusion screens and the like. Thisturbulence is always of the scale of the duct and is as brief aspossible to achieve the desired flow correction.

Similarly, when mixing devices are employed for a specific application,eg. sorption of a particular pollutant, they are usually devices thatgenerate a large-scale turbulence field (i.e. of the order of the ductwidth or height), and are arranged as a brief curtain/s that the gasesmust pass through.

It is also known to use vortex generators in mixing chambers to promotemixing of fluids. Again, the known vortex mixers create large scaleturbulence of the order to the dimensions of the duct or chamber.

Whether they be particulate (e.g. flyash), gaseous (e.g. SO₂), mist(e.g. NO_(x)), or elemental (eg. Mercury), the pollution species whichare the more difficult to collect within industrial exhaust flues arethose of the order of micrometers in diameter (i.e. 10⁻⁶ metres). Due totheir small size, they occupy a very small volumetric proportion of thetotal fluid flow. For example, one million 1 μm diameter particles wouldoccupy less than 0.00005% of the volume of 1 cm³ of gas (assuming thatthe particles are spherical). Even at 10 μm diameter, this proportiononly increases to 0.05%. When it is considered that a pollutant such asMercury may only account for a few parts per million (ppm) of the totalspecies present, it is apparent that relative to particle size, there isa significant amount of space/distance between the species beingtransported by an industrial flue gas. Large scale mixing, even byvortex generators, is therefore a “hit or miss” affair, and largelyinefficient.

Furthermore, it is a characteristic of small particles entrained in aflowing fluid that they will follow streamlines in the fluid flow ifthere is insufficient force to move them out of that flow. That is, ifthe viscous forces of the fluid dominate the inertial forces of theparticle, then the particle will follow the fluid. Known turbulentmixing regimes of the scale of the duct are many orders of magnitudelarger than the particle size. When viewed from the perspective of theparticle, they are far from being chaotic but rather, are relativelysmooth. Whilst there may be many changes of direction for a particle inits passage through a turbulent flow in a duct or through a standardmixing region, they are all relatively long range compared to the sizeor scale of the particle. Consequently, particles in the stream followmore or less the same path without interaction with the particlessurrounding them. At particle scale, there is relatively little mixingand consequently, the known mixing processes achieve poor efficiency inagglomeration.

Systems intended to maximise the collision rate of very small pollutionspecies which occupy a tiny proportion of the volume of the total fluidflow should therefore impart small scale turbulence, i.e. at the scaleof the particle, to have maximum effect. Particle scale turbulence willcause the minute particles to move along many different trajectories atvarious velocities, and thereby promote interactions and agglomeration.Unfortunately, current design philosophies do not adequately addressthese criteria.

It is an aim of the present invention to provide method and apparatusfor mixing fluids for particle agglomeration, to achieve improved mixingor interaction of fine particles in fluid flows, either with the samespecies or other introduced species of larger particles, and therebypromote more efficient agglomeration of the particles or sorption by thelarger particles.

SUMMARY OF THE INVENTION

In one broad form, the present invention provides a method of promotingmixing of substances in a fluid stream, comprising the steps of

generating large scale turbulence in the fluid stream;

dividing the fluid stream into a plurality of substreams;

providing a formation in each substream to create a zone of small scaleturbulence in the vicinity of the formation; and

causing each substream to pass through its respective zone of smallscale turbulence so that it subjected to the small scale turbulence.

In another form, the invention provides apparatus for promoting mixingof substances in a fluid stream, comprising

a conduit for the fluid stream;

a plurality of passages in the conduit for dividing the fluid streaminto substreams flowing through respective said passages;

means for generating large scale turbulence in the fluid stream upstreamfrom the plurality of passages; and

a formation in each passage for generating a zone of small scaleturbulence in the vicinity of the formation;

wherein in use, the large scale turbulence causes the substream in eachpassage to pass through the zone of small scale turbulence.

Each formation is preferably located centrally relative to itsrespective substream, and may suitably comprise a plurality of spacedvanes arranged successively in a plane extending in the overalldirection of flow of the fluid stream.

The vanes should be spaced apart, yet close enough to provide acontinuous zone of small scale turbulence. The vanes can be mounted in agenerally planar frame positioned in a central plane of the passage andextending in the overall direction of flow of the fluid stream.

Each vane is typically an elongate member having sharp edge portionsangled obliquely to the overall direction of flow of the fluid stream.The vane may optionally have a toothed edge portion.

The agglomerator may include a plurality of parallel, generally planar,members extending in the overall direction of flow of the fluid stream,and spaced transversely across the conduit. The passages are definedbetween adjacent pairs of the planar members. However, it is to beunderstood that the passages need not be formed by solid dividers, andmay instead be notional passages for the respective substreams.

In one embodiment of the invention, the conduit is an air duct, thefluid stream is an exhaust gas flow from an industrial process, and thesubstances include pollutant particles. In this embodiment, theinvention involves the use of turbulence to manipulate the position,velocity and trajectories of pollutant particles of micron or sub-micronsize carried in the exhaust gas stream, to increase the probability oftheir colliding with each other and/or with other particles in the gasflow to agglomerate into larger, more easily removable particles, and/orto increase the probability of their colliding and interacting with alarger species of particles introduced into the gas flow for the purposeof removing the pollutant particles.

This process involves the fundamental steps of:

(i) generation of large scale turbulent flows of the appropriate scaleto cause macro turbulence in the exhaust gas stream;

(ii) dividing the gas stream into substreams in respective passages; and

(iii) subjecting the substreams to small scale turbulence.

The terms “large scale turbulence” and “macro turbulence” are intendedto mean turbulence on a scale of the order of the duct dimensions, i.e.turbulence whose influence extends across the entire duct.

The terms “small scale turbulence”, “micro turbulence” and “particlescale turbulence” are intended to mean turbulence on a sufficientlysmall scale to entrain individual particles in the turbulence, andthereby enhance aerodynamic particle agglomeration. This turbulence isnormally restricted to a zone in the immediate vicinity of the vanes.

In the zone of small scale turbulence, which typically extendslongitudinally along a central portion of each passage, the particlesare fully entrained and subjected to turbulent flow. This turbulent flowpromotes collisions and interactions between the small particles,resulting in their agglomeration.

The upstream large scale turbulence is normally caused by the geometryof the conduit itself, e.g. bends, branches, contractions andexpansions. However, if there is insufficient large scale turbulence inthe fluid stream where it enters the passages, additional large scaleturbulence may be imparted to the fluid stream by introducing obstaclessuch as posts and deflectors in the conduit upstream from the passages.

When the turbulent fluid stream is divided into substreams in therespective passages, the substreams are also subject to this large scaleturbulence. Consequently, the particles in each substream passes throughthe zone of small scale turbulence in its respective passage, and aresubjected to micro turbulence, i.e. at particle scale.

The use of small scale turbulence is counterintuitive. Normally, it isdesirable that the pressure drop in the gas stream be as low aspossible. For this reason, known particle mixing systems normally uselarge scale turbulence. However, as mentioned above, these areinefficient. Small scale turbulence promotes better mixing of theparticles, but results in significant pressure loss. The presentinvention employs small scale turbulence but only in a limited zone ineach passage, thereby minimising pressure loss. The large scaleturbulence in the fluid substream in each passage ensures that theparticles in each substream pass through the zone and are subjected tomixing at particle scale.

The small scale turbulence may be in the form of vortices generated bysharp-edged vanes. Preferably, a multiplicity of small, low intensityvortices are used to fully entrain the individual fine particles andsubject them to turbulent flow, thereby resulting in collisions andinteractions between the particles, and more efficient agglomeration ofthe particles. Small particles can agglomerate with each other to formerlarger particles. Small particles can also agglomerate with largerparticles in the fluid stream, The agglomerated particles aresubsequently easier to remove from the gas stream using known methods.

In another embodiment, one or more species of larger particles areintroduced into the gas stream for removal of the pollutant particles.When the pollutant particles contact the larger species, they tend toadhere thereto or react therewith, and can therefore be removed from thegas stream with the larger species. The fine pollutant particles areentrained in the vortices in the zone of small scale turbulence, but thelarger particles in each substream are not, or are entrained to a lesserextent. The relative movement between the small and large particlesresults in higher frequency of collisions between them, and moreefficient removal of the fine (pollutant) particles by the larger(removal) particles.

Preferably, the Stokes number of the small scale turbulent flowgenerated by the vortices is selected so that fine pollutant particleswill be entrained, but not the larger removal species. Typically, aStokes number much less than 1 will ensure entrainment of the finepollutant particles. The larger removal species of particles should havea Stokes number much greater than 1 so that they are not entrained. Inpractical terms, the eddies or vortices generated in the gas stream areof the order of 10 mm.

The pollutant particles may be of gaseous, liquid or solid form. Thelarger species may be of liquid or solid form, e.g. liquid droplets.

The removal species may be a chemical, such as calcium, which reactschemically with pollutant particles, (such as sulphur dioxide) to form athird compound (e.g. gypsum). Alternatively, the removal species ofparticles may remove the pollutant particles by absorption, or byadsorption (carbon particles adsorbing pollutant mercury particles), orthe removal species of particles may simply remove the fine pollutantsby agglomerating with the pollutants through impact adhesion.

In order that the invention may be more fully understood and put intopractice, embodiments thereof will now be described, by way of exampleonly, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a duct having an agglomerator according to oneembodiment of the invention.

FIG. 2 is a plan view of the agglomerator of FIG. 1.

FIG. 3 is a schematic sectional plan view of a portion of a vaneassembly of the agglomerator of FIG. 1.

FIG. 4 is a perspective view of a vane of the vane assembly of FIG. 3.

FIG. 5 is a schematic sectional plan of part of the agglomerator of FIG.1, showing large scale turbulence.

FIG. 6 is a schematic sectional plan of a portion of a vane assembly ofFIG. 3, showing regions of small scale turbulence.

FIGS. 7(a) to (e) are perspective views of alternative vanes.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 to 6 illustrate an aerodynamic agglomerator according to oneembodiment of this invention. The agglomerator 10 is housed in a duct 11which typically receives a flow of exhaust gas from an industrialprocess, as shown in FIG. 1.

The agglomerator 10 comprises a plurality of generally planar members,such as metal plates 12, which extend longitudinally in the duct 11(i.e. in the direction of overall gas flow), and are spaced transverselyacross the whole width of the duct. Passages are formed between theplates 12, and the gas flow is divided into substreams flowing throughrespective passages. Although the plates 12 are mounted vertically asshown in FIG. 2, they can be arranged horizontally if desired. Moreover,the plates 12 need not be solid. Perforated plates can be used ifdesired.

Vane assemblies 13 are mounted between the plates 12. Each vane assembly13 is located centrally in its respective passage between two adjacentplates 12, and extends parallel to the plates 12 as shown more clearlyin FIG. 5.

The construction of each vane assembly 13 is shown in more detail inFIGS. 3 and 4. Each vane assembly 13 comprises a generally planarrectangular frame 14 which, in use, may be suspended from the duct roofcentrally in the passage between a pair of adjacent plates 12. Eachframe 14 has a plurality of spaced upright vanes 15 mounted generallywithin the plane of the frame. Each vane 15 is typically a metal stripof “Z” section, angled to the direction of gas flow through the passage.The vertical edges 17 of each vane 15 are preferably scalloped to formteeth 16 having a depth T_(d), and a spacing or pitch T_(p).

The vane length V₁ is the dimension of the main body of the vane 15 inthe direction of gas flow, as shown in FIG. 3. The vane spacing V_(s) isthe distance between successive vanes, excluding teeth. The vane widthV_(w) is the dimension of the main body of the vane 15 transverse to thegas flow. The passage width P_(w) is the internal distance or spacingbetween adjacent plates 12.

Sufficient plates 12 are provided to divide the full width of the duct11 into passages, and sufficient vane assemblies 13 are provided so thata vane assembly is position centrally in each passage between adjacentplates. Typically, the passage width is around 275 mm, but passagewidths may typically range from 100 mm to 750 mm, providing that theratio of passage width P_(w) to vane width V_(w) is maintained between aminimum of 2.5 and a maximum of 25.

The vanes 15 in each frame 14 are spaced longitudinally, so thatsuccessive vanes are in the flow wake or shadow of the preceding vane.The spacing V_(s) between successive vanes 15 is roughly equivalent tothe size of the flow wake generated by the leading vane. In this manner,there is overlap between the microturbulence generated by adjacentvanes, or at least a continuous region of microturbulence.

The flow wake generated by a vane 15 is proportional to the width V_(w)of the vane in the direction transverse to the gas flow, and the lengthV₁ of the vane in the direction parallel to the gas flow. In theillustrated embodiment, V_(s) is approximately equal to V₁. The vanespacing Vs may suitably range from 0.5 V_(w) to 8 V_(w). Similarly, thevane length V₁ may suitably range from 0.5 V_(w) to 8 V_(w).

If teeth are used on the vanes, the tooth depth is typically 0.25V_(w)to 2 V_(w), and the tooth pitch is typically 0.5 V_(w) to 2 V_(w).

It is to be noted that the agglomerator 10 is passive, i.e. thecomponents of the agglomerator are not charged or electrified to anysignificant extent.

In use, the gas flow in the duct 11 will be subjected to large scale ormacro turbulence. Ordinarily, the presence of expansions, contractions,bends, branches, deflectors, vanes, braces and other physical formationscommonly found in industrial exhaust ducts will be sufficient to impartthe large scale turbulence to the air flow. For example, deflector vanes18 used to direct gas flow cause separation and long range turbulence inthe gas flow. If however, there is insufficient macro turbulence in thegas stream when it reaches the agglomerator 10, flow disrupters can beadded to the duct 11 to provide the necessary macro turbulence. Forexample, if there is a significant length of duct (say, equivalent tofour duct diameters) immediately prior to the agglomerator 10 which isfree of turbulence inducing formations, then flow disrupters should beadded to the duct.

A suitable flow disrupter is an array of 100 mm diameter pipes 9 (oralternatively 100 mm×100 mm angle sections) mounted in the duct 11 sothat they extend fully through the gas stream to cause large scaleturbulence. Such pipes 9 should be mounted no more than 1 metre apartacross the duct. It will be apparent to those skilled in the art thatmany different physical formations can be used upstream of theagglomerator 10 to impart macro turbulence to the gas stream if there isinsufficient large scale turbulence immediately prior to theagglomerator 10.

When the gas flow passes through the agglomerator 10, it is divided intosubstreams which flow through respective passages between adjacentplates 13. The macro turbulence in the gas stream continues in thesubstreams, causing the particles in each substream to pass through thevane assembly 13 in the corresponding passage, as illustrated by theflow lines in FIG. 5. The large scale, long range turbulence in thesubstreams ensures that substantially all of the substream in a passagecirculates through the vane assembly 13 located centrally in thepassage.

When a substream passes through a vane assembly 13, it is subjected tosmall scale or micro turbulence, as indicated by the shaded portions 19in FIG. 6. The angled vanes 15 create turbulence at particle scale,promoting interactions and collisions between particles in the substreamwithin each passage, and enhancing the agglomeration of the particles.Due to the small scale turbulence created in the vicinity of the vanes11, particles in the substream are entrained in the turbulence, leadingto significantly increased likelihood of collision and adherence. Theadherence process may be a surface interaction (such as an adsorption,chemisorption or absorption process), a molecular interaction (as aresult of van der Waals forces) or a wetting process (as a result of theimpact of mists with other mist droplets or solid particles).

The small scale or micro turbulence may be in the nature of a pluralityof small vortices, typically 10-15 mm. The angled surfaces, sharp edgesand discontinuous or zigzag formations of the vanes 15 act as vortexgenerators, creating a multitude of vortices along each sub-stream.These vortices are of a very small size, and entrain fine pollutantparticles in the gas stream.

The vortex patterns generated by the vanes 15 are believed to include atransverse eddying motion, aligned parallel to the vanes, whosedimensions are dependent upon the vane spacing, the vane length and thevane width, and a series of counter-rotating vortex structures whosedimensions are dependent upon the teeth 16 of the vanes. The flowvelocity around the vanes 15 is believed to be substantially less thanthe mean flow velocity.

Although the zone of micro turbulence is limited to the centre of eachpassage, the macro turbulence in each substream ensures that thesubstream passes through this zone so that the particles in thesubstream are subjected to turbulence at particle scale. Moreover, bylimiting the small scale turbulence to the centre region of eachpassage, the overall pressure drop through the agglomerator isminimised.

The foregoing describes only one embodiment of the invention, andmodifications which are obvious to those skilled in the art may be madethereto without departing from the scope of the invention as defined inthe accompanying claims. For example, although the invention has beendescribed with particular reference to the mixing of particles in a gasstream, it also has application to mixing in other fluid flows, e.g.liquids.

Furthermore, the shape and configuration of the vanes can be varied.FIGS. 7(a) to (e) illustrate alternative forms of vanes which may beused in the illustrated agglomerator.

Although the vanes 15 are preferably provided with teeth 16 to intensifythe micro turbulence and focus it in the region immediately downstreamof the vane, they are not essential to its creation. The zone of smallscale turbulence can be generated by any suitably shaped vane (e.g.rods, bars, fins, etc), and will be concentrated between successivevanes if the vanes are aligned one behind the other in the wake of thepreceding vane and are spaced so that the wake can fully form betweensuccessive vanes.

1. A method of promoting mixing of substances in a fluid stream,comprising the steps of generating large scale turbulence in the fluidstream; dividing the fluid stream into a plurality of substreams;providing a formation in each substream to create a zone of small scaleturbulence in the vicinity of the formation; and causing each substreamto pass through its respective zone of small scale turbulence so that itsubjected to the small scale turbulence.
 2. A method as claimed in claim1, wherein each formation is located centrally relative to itsrespective substream.
 3. A method as claimed in claim 2, wherein theformation comprises a plurality of spaced vanes arranged successively ina plane extending in the overall direction of flow of the fluid stream,the vanes being spaced close enough to provide a continuous zone ofsmall scale turbulence.
 4. A method as claimed in claim 1, wherein thefluid stream is an exhaust gas flow from an industrial process, and thesubstances include pollutant particles.
 5. A method as claimed in claim4, wherein the substances include particles added to the fluid stream toagglomerate with the pollutant particles.
 6. A method as claimed inclaim 1, when the step of dividing the stream into a plurality ofsubstreams comprises directing the stream into a plurality of passagessuch that each substream flows through a respective passage. 7.Apparatus for promoting mixing of substances in a fluid stream,comprising a conduit for the fluid stream; a plurality of passages inthe conduit for dividing the fluid stream into substreams flowingthrough respective said passages; means for generating large scaleturbulence in the fluid stream upstream from the plurality of passages;and a formation in each passage for generating a zone of small scaleturbulence in the vicinity of the formation; wherein in use, the largescale turbulence causes the substream in each passage to pass throughthe zone of small scale turbulence.
 8. Apparatus as claimed in claim 7,wherein each formation is located centrally relative to its respectivepassage, and its generated zone of small scale turbulence is located inthe vicinity of the formation.
 9. Apparatus as claimed in claim 8,wherein each formation comprises a plurality of spaced vanes arrangedsuccessively in a plane extending in the overall direction of flow ofthe fluid stream.
 10. Apparatus as claimed in claim 9, wherein the vanesof the formation in each passage are mounted in a generally planarframe, the frame being located substantially centrally relative to thepassage and extending in the overall direction of flow of the fluidstream.
 11. Apparatus as claimed in claim 9, wherein each vane is anelongate member having sharp edge portions angled obliquely to theoverall direction of flow of the fluid stream.
 12. Apparatus as claimedin claim 11, wherein each vane has a toothed edge portion.
 13. Apparatusas claimed in claim 7, further comprising a plurality of parallel,generally planar, members extending in the overall direction of flow ofthe fluid stream, and spaced transversely across the conduit, thepassages being defined between adjacent pairs of the planar members. 14.Apparatus as claimed in claim 7, further comprising additionalformations in the conduit upstream of the passages for promoting largescale turbulence in the fluid stream.
 15. Apparatus as claimed in claim7, wherein the conduit is an air duct, the fluid stream is an exhaustgas flow from an industrial process, and the substances includepollutant particles.
 16. A non-energised aerodynamic agglomerator forpromoting mixing and agglomeration of pollutant particles in a gasstream, the agglomerator comprising a duct for receiving the gas stream;a plurality of parallel, generally planar, members mounted in the duct,the planar members extending in the overall direction of flow of the gasstream, and being spaced transversely across substantially the wholewidth of the duct, each adjacent pair of the planar members defining apassage between them; the duct being configured and/or having flowaltering members therein for promoting large scale turbulence in the gasstream upstream of the passages; a formation in each passage forgenerating a zone of small scale turbulence of such size and/orintensity that the pollutant particles are entrained in the turbulence,each formation being located centrally relative to its respectivepassage and comprising a plurality of spaced sharp-edged vanes arrangedsuccessively in a plane extending in the overall direction of flow ofthe gas stream; wherein in use, the gas stream is divided into aplurality of substreams flowing through the respective passages, and thelarge scale turbulence in the substreams causes each substream to passthrough the zone of small scale turbulence its respective passage sothat particles therein are subjected to the small scale turbulence.